The present disclosure relates to an optical pickup and an optical disc device that are employed to record onto and to play back a multilayer optical disc.
An optical disc device that records data onto or plays back data on various optical discs having a spiral track such as a Blu-ray Disc (BD)™ is known. In such an optical disc device, a track in an optical disc is irradiated with a light beam when recording or playing back data. At that time, the optical disc device generates a tracking error signal that indicates an amount of deviation from the center of the track to be irradiated with the light beam, based on reflected light from the optical disc. Then, the optical disc device carries out tracking control based on the tracking error signal so that the light beam constantly stays on the track.
A differential push pull (DPP) method is known as a tracking control method. In the DPP method, a laser beam from a laser light source is split into a main beam and two sub-beams using a grating. Then, a tracking error is detected from a sub-beam push pull (SPP) signal, and a DC offset that occurs in a main beam push pull (MPP) signal is canceled.
There is a growing demand for increased capacity of an optical disc. Multilayering in which a plurality of signal faces is stacked on one another is one of the methods for increasing the capacity of an optical disc. When recording data onto or playing back data on such a multilayer optical disc, a light beam is focused on a desired signal face.
However, since a light beam passes through signal faces other than a signal face that is most distanced from the surface of the multilayer optical disc on which the light beam is incident, the light beam is reflected by the signal faces of layers (hereinafter, referred to as other layers) other than a desired layer on which recording or playback is carried out (hereinafter, referred to as a recording/playback layer). Reflected light from the signal faces of the other layers (referred to as other layer stray light) is incident on a photodetector as crosstalk light. Then, noise is generated as the other layer stray light is superimposed onto the reflected light from the signal face of the recording/playback layer (hereinafter, referred to as signal light).
The influence of the other layer stray light is greater in a BD than in a multilayer digital versatile disc (DVD). That is because the numerical aperture (NA) of an objective lens is larger for a BD, and thus the spot size of the crosstalk light is larger. In the case of the DPP method, an SPP with less power is affected more by the crosstalk light. Accordingly, a push pull method of a single beam system is used. In the push pull method, light receiving signals are detected by a photodetector having a light receiving face that is divided into two regions along a track direction (tangential direction), and a tracking error is detected from a difference in the light receiving signals in the respective regions.
In the push pull method, a push pull signal is formed based on diffracted rays that are diffracted by grooves of different depths. If the groove depth is as small as λ/12, a tracking error signal to be obtained through calculation has small amplitude. Thus, there has been a problem in that an offset occurs in a tracking error signal around a boundary between a recorded area and an unrecorded area. In order to solve such a problem, a method called an advanced push pull (APP) is being proposed for detecting a tracking error signal. In addition, an improved APP method that can be applied to a dual layer disc is being proposed (see, for example, Kousei Sano et al., “Novel One-Beam Tracking Detection Method for Dual-Layer Blu-ray Discs,” IEICE Technical Report, CPM, Oct. 14, 2005, vol. 105, No. 360, pp. 31-34).